Hot-swap is a useful function for inserting a hard disk drive, an interface card into a system, and removing a device from the system when the system is hot without doing harm to the system. Prior art uses a FET as a hot-swap, which operates as a LDO (low dropout) at start-up, as shown in FIG. 1a. 
FIG. 1a schematically shows a prior art power system 50 with hot-swap. The power system 50 comprises: a front stage 51 including a front capacitor CIN, wherein the front stage 51 is configured to provide a power supply VIN; a hot-swap stage 52 coupled to the front stage 51 to receive the power supply VIN, and to provide an output voltage VO based thereupon; and a load stage 53 including an output capacitor CO coupled to the hot-swap stage 52 to receive the output voltage VO. Typically, the hot-swap stage 52 and the load stage 53 are placed at a board. In the example of FIG. 1a, the hot-swap stage 52 comprises a MOSFET and a controller, wherein the MOSFET is controlled to operate at LDO mode by the controller when the board is plugged to the front stage 51 (i.e., at start-up), to generate the output voltage VO with gradual increase from zero.
FIG. 1b schematically shows the waveforms of the output voltage VO, a voltage drop of the MOSFET VDS, a current flowing through the MOSFET IS, and the power of the MOSFET P—MOS in the hot-swap system 50 in FIG. 1a. Since the MOSFET operates at LDO mode, the output voltage VO increases gradually from zero to the input voltage value minus the saturated voltage drop of the MOSFET. However, the power supply VIN is constant, so the voltage drop of the MOSFET VDS decreases gradually from the value of the power supply to its saturated voltage. In addition, during the start-up of the plugging of the board, the load stage 53 operates as a current source to sink the current, while the current flowing through the MOSFET includes both the load current and the current flowing through the output capacitor CO. Thus, the MOSFET consumes a large power loss. Although this power loss is minor since the start-up time is short in the whole operation of the power system, the thermal design has to follow the power loss condition of start-up. Normally, for high current application, several MOSFETs and large board space for thermal dissipation are required. So it is thermally over-design due to this large power loss at start-up compared to the system operates at steady status for most of time.